The present invention relates to methods of coating a supporting material with a liquid-state coating compound (hereinafter referred to as "a coating liquid" when applicable). More particularly, the invention relates to a method of simultaneously applying the multiple layers of coating liquids on a moving flexible belt-shaped material (hereinafter referred to as "a web" when applicable).
A coating method of this general type is well known in the art and is described in Japanese Laid-Open Patent Application No. 115214/1977. The specific feature of the conventional coating method resides in that the web is run at least at a speed of 100 cm/sec, the lowermost layer is thin and is formed by a coating compound of low viscosity, a layer immediately above the lowermost layer is thicker than the lowermost layer and is formed by a coating compound of high viscosity, and the thickness and viscosities of the lowermost layer and the layer immediately above the lowermost layer are selected so that the eddy movement of the coating bead is limited in the two layers. With this arrangement, while inter-layer mixing occurs between the two layers, the remaining layers are formed individually without mixing during the coating process. The viscosity and the covering power of the coating liquid forming the lowermost layer are made lower than those of the layer next to the lowermost layer to decrease the load required for fully drying the layers thereby increasing the coating speed.
However, the conventional coating method is disadvantageous in that, as the viscosity of the coating liquid forming the lowermost layer is low, the bead region is liable to become unstable and, accordingly, the ability to withstand high speed coating is very small.
On the other hand, in U.S. Pat. No. 4,113,903 there is proposed a coating method in which double-layer liquid coating is employed which includes the application of the shearing thinning carrier layer of a pseudoplastic liquid as a layer next to the web with the pseudoplastic liquid having a viscosity of from 20 cp to 200 cp at a shear rate of 100 sec.sup.-1 and a viscosity lower than 10 cp at a shear rate of 100,000 sec.sup.-1. That is, the conventional coating method uses a pseudoplatic liquid which has a high viscosity at low shearing rates and a low viscosity at high shearing rates to eliminate the above-described instability of the bead region.
However, the conventional method is still disadvantageous in the following points. Even with the physical properties described above maintained only for the coating liquid for forming the lowermost layer, the coating liquids forming the lowermost layer and the next layer become ill-balanced in the physical property of fluidity. As a result a so-called "interlayer mixing and waving" phenomena occur as the coating liquids flow down the slide surface of a slide hopper type coating apparatus. Accordingly, it is difficult to produce webs of satisfactory quality.